This news isn't really surprising. First of all, reports have
pointed to Nintendo debuting and releasing a brand new game
console this year —
the mysterious NX — so it makes sense that Nintendo would
also stop producing its Wii U console around that same time.
There's also a good chance the NX can play Wii and Wii U games,
so that would effectively make the Wii U hardware obsolete.

Need another reason? Here's a big one: The Wii U just isn't
selling. People aren't buying this console — just look at this
evidence straight from Nintendo.

Nintendo

As you can see, the Wii U is the worst-selling console Nintendo
has ever created (save for possibly the Virtual Boy, which isn't
listed here). The Wii U's sales are roughly one-tenth the
lifetime sales of Nintendo's previous console, the Wii. And it's
only reached about half of the lifetime sales of the Nintendo
Gamecube, which was the company's second-worst selling console.